Many methods are known for the preparation of alkylisothiocyanic acid esters, also designated as alkylisothiocyanates or alkyl mustard oils--cf. Ullmanns Enzyklopadie der technischen Chemie, 4th edition, volume 23, pp. 154-159 and Published German Patent Application DE-OS 21 05 473. With the exception of the method known from DE-PS 21 05 473, none of the previously known methods found acceptance in the art, either because the preparation of the raw materials was too expensive or because the yields of pure alkylisothiocyanates were too low.
The method of DE-OS 21 05 473 for the preparation of methylisothiocyanate, which is used as an intermediate product for organic syntheses as well as increasingly as a nematocide and a fungicide, did achieve industrial significance. The method of DE-OS 21 05 473 was applicable to the preparation of alkylisothiocyanic acid esters of the general formula R--N.dbd.C.dbd.S, in which R stands for a lower alkyl group. In that method, one mole of an aqueous solution of an N-alkyldithiocarbamate of the general formula R-NH-C(S)-SMe, in which R has the previously indicated significance and Me can stand for an alkali-metal atom, an ammonium group or alkylammonium group, is reacted at a temperature of 50.degree. to 120.degree. C. with at least 1 mole of a 20 to 70% by weight aqueous solution of hydrogen peroxide while maintaining a pH of 5 to 9. Then the alkylisothiocyanic acid ester which is formed is isolated in a known manner. According to the claims of DE-OS 21 05 473, it was preferred to use 2.3 to 2.5 moles hydrogen peroxide for each mole of dithiocarbamate and a pH of 6 to 8 was maintained; the use of a dithiocarbamate solution which was as concentrated as possible was considered to be especially advantageous. Half a mole sulfur and half a mole sulfate are produced as byproducts in this method at a molar ratio of dithiocarbamate to hydrogen peroxide of approximately 1 to 2.5 per mole alkylisothiocyanate. Whereas these byproducts can be separated readily on a laboratory scale when the method is carried out, the separation of the sulfur poses problems in large-scale industrial production; the sulfur accumulates in part as hard lumps, and, in addition, the reactor can become encrusted. Frequent and often lengthy interruptions of the operation are necessary for removing the lumps and encrustations. This considerably reduces the space-time yield of the method.